The present invention relates generally to the stabilizing of an alternating current (ac) synchronous generator and more particularly to the damping of oscillations of a synchronous generator.
In large ac power networks, the speeds of the several ac generators connected thereto can oscillate about the base system frequency (e.g., 60 hertz). As such, although the average system frequency is constant, some generators may be running above synchronous speed while, simultaneously, other generators are running below synchronous speed. The speed of a given generator can oscillate above or below its synchronous speeds such that its period of oscillation is in the approximate range of 0.2 to 2.0 seconds. Since, at a given instant, the generators may be running at different speeds and are, consequentially, generating different frequencies, appreciable voltages can exist across any connecting impedance such as the transmission lines, transformers, etc. These voltages can, in turn, result in excessive currents which may cause circuit breakers to open and thus interrupt electric service to users of electric power. It has, therefore, been customary and desirable to provide some means to dampen the oscillations in generator speed to enhance reliability of electric service.
Damping of the generator oscillations in a large power system is typically provided by what is called a power system stabilizer (PSS). A PSS is an auxiliary control device used in conjunction with the generator excitation system. The details and operation of a power system stabilizer are well known to those skilled in the art and the various types are presently in use. In general, however, the PSS senses generator terminal voltage and terminal current and in response thereto produces a signal which represents the instantaneous magnitude of the generator oscillations which latter signal is used to control the generator excitation such that a damping torque is applied to the generator rotor. Examples of systems employing a PSS include U.S. Pat. No. 4,741,023 "On-Line Test and Diagnostic System for Power System Stabilizer" by Rodney A. Lawson and U.S. Pat. No. 4,463,306 "System for Stabilizing Synchronous Machines" by F. P. de Mello et al. and reference may be had to these patents, which are hereby incorporated by reference, for further explanation as to the details of a PSS.
Typically, the field winding of the generator is energized by an exciter which is under the control of an automatic voltage regulator. In many of these systems, the output signal of the PSS is fed to a summing point within the voltage regulator and serves to modulate the voltage regulator output to control generator oscillations. In some voltage regulators, however, there either is no such summing point or it may be inaccessible to outside signals. For those types of voltage regulators where the PSS signals cannot be directly applied, it is known that the appropriate signal can be injected by means of a modulator. An example of such modulator is found in the de Mello et al. patent previously cited. The modulator in that patent consists of an autotransformer having taps which are selected by solid state switches. These switches are controlled by the PSS output signals. The system of the de Mello et al. patent suffers from a major drawback in that, because of the tap switching, the output is a step function rather than being continuously variable. In addition, the requirement for a large number of switches, even though solid state, adds degrees of complexity and potential failure points to the overall system.